1. Field of the Invention
The present invention relates to image forming apparatuses such as copying apparatuses, printers, etc., that carry out image forming using the electrophotographic method, and in particular, to image forming apparatuses that carry out fixing of unfixed toner images using a fixing unit by applying heat and pressure.
2. Description of Related Art
In an image forming apparatus, the toner images formed on an image bearing body (photoreceptor) or on an intermediate transfer body are transferred by an image transfer section on to a recording material (also called a transfer material), and the recording material carrying the toner image is subjected to heat and pressure when it passes through the fixing nip section formed by a heat fixing member and a pressure member of the fixing unit that are in a pressure contact state, and the toner gets fixed on to the recording material due to adhesion.
During such fixing, there is a phenomenon called blistering that is not desirable for image formation. Blistering is a phenomenon that occurs mainly when coated paper with a low basis weight is used as the recording sheet, and is a phenomenon in which image blemishes appear with the surface of the toner layer becoming textured because, while the coated paper passes through the nip section of the fixing unit, the air or moisture in the toner layer cannot escape to the outside but become bubbles within the toner image, and also because the moisture in the coated paper passes through the coating layer and evaporates.
While the coated paper is passing through the nip section comprising a heat fixing member which is, for example, a fixing belt, and a pressure member which is, for example, an external pressure roller, the toner image is fixed by melting on top of the recording material, and while it is necessary to take the toner layer to a sufficiently high temperature in order to obtain the necessary glossiness, because of this heating, the recording material and the layer below the toner image that does not contribute to the glossiness are also heated more than is necessary, thereby causing the blistering phenomenon. It has been known that the temperature of the pressure member has a large influence on the temperature of the recording material and of the layer below the toner image, and the generation of blisters is reduced by maintaining the temperature of the pressure member at a low value.
However, since the external pressure roller is in pressure contact with the fixing belt during image formation, the temperature of the external pressure roller exceeds the permissible temperature due to the heat received by it from the fixing belt through the paper interval or outside the sheet passing region even though it is not heated intentionally, thereby resulting in an environment of generating blisters.
In Patent Document 1, as a means for preventing the generation of blisters, a configuration has been disclosed in which the set temperatures of the heat fixing roller which is a heat fixing member and of the endless belt shaped pressure member are set at different values, and, based on the respective detected temperature conditions that have been detected using sensors, the heat fixing roller and the pressure member are rotated respectively in a state in which they are separated from each other.
In the disclosure made in Patent Document 2, in order to prevent wrinkling of the transfer material or disturbances in the fixed image, it has been disclosed to arrange four cooling fans in the axial direction of the pressure roller, these fans are driven according to the size of the sheet, thereby lowering the temperature of the high temperature parts of the pressure roller.
In the disclosure made in Patent Document 3, as a method of shifting the optimum amount of heating of the fixing roller from the high heating amount to low heating amount, a cooling fan that cools the periphery of the fixing roller, and when the temperature of the fixing roller becomes higher than the set fixing temperature range, based on the temperature detection signal from the temperature detecting means, the control means drives the cooling fan thereby lowering the temperature of the entire fixing roller uniformly.
Patent Document 1: Unexamined Japanese Patent Application Publication No. Hei 11-194647
Patent Document 2: Unexamined Japanese Patent Application Publication No. Hei 6-242701
Patent Document 3: Unexamined Japanese Patent Application Publication No. Hei 10-20707
In order to prevent the generation of blisters, the method in Patent Document 1 of separating the heat fixing member and pressure member is not desirable because during continuous printing, the problem that the printing has to be interrupted in order to separating these two members, etc.
Further, in order to cool the external pressure roller, the method of cooling by applying an air blow from outside the roller using a cooling fan as disclosed in Patent Documents 2 or 3 causes heat pollution because the hot air gets dispersed within the equipment after cooling.
Apart from using a cooling fan, although there is the method of cooling the surface of the roller by making a heat pipe come into contact with the external pressure roller, it is difficult to obtain sufficient cooling effect because a temperature difference is necessary between the surface of the roller and the heat pipe in order to carry out cooling.
In order to cool the external pressure roller, when detecting the temperature of the external pressure roller and then starting the cooling using these cooling means, there will be a delay before the cooling effect appears due to the thermal capacity of the roller itself and the cooling may not be done in time.
The purpose of the present invention is to provide a color image forming apparatus provided with a fixing unit that controls the dispersion of heat within the apparatus at the time of cooling the external pressure roller to the minimum level and suppresses the temperature rise of the external pressure roller so that the generation of blisters cannot be observed.